1. Field of the Invention
The invention relates to an apparatus and process for reducing speckle exhibited by a laser beam, and particularly to insertion of an anti-speckle apparatus in the path of a laser beam in order to control the spatial coherence of the beam independent of the construction of the laser resonator.
2. Discussion of the Related Art
Radiation in the deep ultraviolet (DUV, λ>200 nm) and vacuum ultraviolet (VUV, λ<200 nm) is currently being applied in photolithography to fabricate integrated circuits. Specifically, the output of narrow band ArF and KrF excimer lasers (λ=193 nm, 248 nm) are utilized for making semiconductor devices having a feature size of between about 0.18 and 0.25 μm (produced by radiation from a KrF laser), or between about 0.13 and 0.18 μm (produced by radiation from a ArF laser). The molecular fluorine laser emitting around 157 nm is also used for forming small structures. EUV sources such as plasma focus sources emitting around, e.g., 13 nm will come into prominence in the future.
Because of the extremely small magnitude of these dimensional ranges and their size relative to the wavelength of the incident laser beam, achromatic imaging optics are difficult to achieve in these wavelength regions. In order to reduce imaging errors caused by chromatic aberration, excimer lasers must produce radiation of an extremely narrow bandwidth ranging between about 0.3 and 0.6 pm. Bandwidth is generally defined as the full width at one-half the maximum of the spectral line shape.
The narrow bandwidths produced by these lasers in turn cause the beam to exhibit a high degree of spatial coherence. Spatial coherence describes coherence exhibited by the light rays of the beam as measured across a beam cross-section. In contrast, temporal coherence describes coherence exhibited by the light rays of the beam over time as the beam traverses a distance.
Unfortunately, high spatial coherence can create problems relating to speckle. Speckle is an interference pattern created by a slight difference in path traveled by the light rays making up the laser beam. Speckle can result from interaction between the beam and minor environmental elements, for example reflection of the beam by slightly turbulent air or by microscopically rough surfaces. Beams exhibiting a high degree of spatial coherence are especially prone to exhibit speckle. Speckle is detrimental to many laser applications because it causes uneven illumination of areas by the beam. Speckle is thus a particularly serious problem in photolithography, where uneven beam illumination can result in non-uniform development of photoresist materials that define the microscopic feature size of active devices.
In photolithography, a laser is generally employed as one component of a larger optical system such as a stepper or scanner. Due in part to concerns about speckle, stepper and scanner manufacturers impose an upper limit on the spatial coherence of lasers eligible for use in their systems. Unfortunately, these spatial coherence requirements vary from manufacturer to manufacturer.
One way to accommodate spatial coherence requirements of equipment manufacturers is to modify the design of the resonator generating the laser beam. However, this approach is impractical for two reasons.
First, the resonator is a complex structure whose performance and output is extremely sensitive to structural changes. Thus, modification of the resonator to conform to the requirements of every manufacturer of a particular optical system would be expensive and time-consuming.
A second problem is that design of the resonator dictates a number of important characteristics of the laser beam apart from bandwidth and spatial coherence. One such characteristic is the spectral purity.
While bandwidth represents the full width at one-half the maximum of the spectral line shape, the spectral purity represents the wavelength interval containing 95% of the pulse energy of the beam. Lasers used in the next lithography generation will emit beams having a spectral purity of less than 1 pm.
Another important beam characteristic is divergency. Divergency describes the angular spreading of the beam cross-section over distance, and is characterized by an angle θ. For beams produced from a resonator utilizing line narrowing elements such as prisms and gratings but without an etalon, beam divergence is dictated solely by the length of the resonator and intracavity apertures. For beams produced from a resonator featuring an etalon, beam divergence is also determined by the properties of the etalon.
To summarize, the line-narrowed lasers employed in photolithography produce radiation exhibiting comparatively small divergency and a high degree of spatial coherence. The character of this radiation is attributable to the particular design of the resonator producing the beam. As a result of the high degree of spatial coherence of the beam, it is prone to exhibit speckle.
Given the problem posed by speckle and the difficulty in modifying the resonator design, there is a need in the art for an apparatus and method permitting precise control over the spatial coherence of a laser beam independent of the design of a particular laser resonator.